Modeling stress-dependent matrix cracking and stress–strain behavior in 2D woven SiC fiber reinforced CVI SiC composites

Abstract

2D woven Hi-Nicalon and Sylramic-iBN SiC fiber reinforced chemical vapor-infiltrated (CVI) SiC matrix composites were tested at room temperature with modal acoustic emission monitoring in order to determine relationships for stress-dependent matrix cracking. The Hi-Nicalon composites varied in the number of plies (1–36), specimen thickness, and constituent content. The Sylramic-iBN composites were fabricated with balanced and unbalanced 2D weaves in order to vary the fiber volume fraction in the orthogonal directions. Not surprisingly, matrix cracking stresses tended to be, but were not always, higher for composites with higher fiber volume fractions in the loading direction. It was demonstrated that simple relationships for stress-dependent matrix cracking could be related to the stress in the load-bearing CVI SiC matrix. For low-density composites, the 90° minicomposites do not share significant loads and matrix cracking was very similar to single tow minicomposites. For higher-density composites, where significant load is carried by the 0° minicomposites, matrix cracking was dependent on the unbridged “flaw” size, i.e., the 90° tow size or unbridged transverse crack size.